Sulfolene dioxide cheletropic elimination

The most synthetically useful cheletropic elimination involves 2,5-dihydrothiophene-1,1-dioxides (sulfolene dioxides). At moderate temperatures they fragment to give dienes and sulfur dioxide.301 The reaction is stereospecific. For example, the dimethyl derivatives 22 and 23 give the E,E- and Z,E-isomers of 2,4-hexadiene, respectively, at temperatures of 100°-150°C.302 This stereospecificity corresponds to disrotatory elimination. 

The term "cheletropic reaction" defines reactions in which two a bonds to a single atom are made or broken conceitedly. A widely studied example, theoretically as well as experimentally, is the cheletropic elimination of sulfur dioxide from a cyclic sulfolene to generate stereochemically pure dienes or polyenes. ... 

One of the best routes to o-quinodimethanes involves cycloreversion with elimination of a small, thermodynamically stable, molecule. The most important example is the loss of sulfur dioxide in the widely used cheletropic extrusion from sulfolenes (Section 2.4). In this allowed lc 2 + bond order of the "backbone" sulfolene double bond is ideally 2. If the bond order is lowered by bond fixation this renders the extrusion more difficult and the... 

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